Harley Engines: Don't Get Bored with Bores Ever Again

Man, oh man. The weather turned beautiful here in Anamosa—70 degrees, sunshine and just a touch of wind. Bikes are starting to come out from winter storage, and more and more people are itching to get out on the road. As we begin the riding season, let’s dive into what makes up these engines.

Eastwood Engine Bore Gauge

There are so many things happening these days when it comes to engines, it’s hard to keep up. The standard engine rolling off the H-D line is the 103 cubic-inch (ci) Twin Cam, with the CVO line getting a 110 ci Twin Cam.

This is not news. The 103 ci became standard in almost all Big Twins in 2012, and the 110 ci was standard in CVO models back in 2008. Prior to that was the 96 ci Twin Cam and the original Twin Cam 88 ci.

So in a span of 16 years we have seen three major jumps in displacement, which when you think about it, is a good indication of the versatility of the Twin Cam. By comparison, for almost 45 years, from 1936 through 1980, Harley saw only two engine displacements—61 ci for the Knuckleheads and early Panheads, and 74 ci as an option for the Knucklehead, and becoming standard for the Panhead and Shovelhead.

The 80 ci Shovelhead led to the 80 ci Evolution engine, which saw no factory displacement changes, and when you think about it, only one major change, from crankcase breathing to head breathing. (More on engine breathing in a later post.)

So what does this all mean? First, that there are always going to be new, bigger toys speeding out your motorcycle dealer’s driveway. Second, H-D recognized a growing trend to get more out of the standard motor.

So what is it about the Twin Cam that sets it apart? Why do we see so many people ready to dive into this motor and make it larger? And most importantly how easy is it?

Let’s begin with what makes up the engine size. Displacement is bore x bore x stroke x 0.7854 x number of cylinders. This is a standard volume formula used to measure cylinder volume. You can see by the table that the Twin Cam 96 is actually shorting itself.

You see, once the Twin Cam went to the 4.374” stroke on the Twin Cam 96, the ability to grow larger was only limited by the bore size.

You might be asking, “Well, what limits bore size?” A few things to start. First is the thickness of the liner inside the cylinder. This has to remain thick enough to withstand the heat created by friction of the pistons and temperature of combustion, and strong enough to withstand the pressure created in the sealed environment. (Insert any number of hot heads you know that you have seen “blow a gasket.”)

What happens when the walls get too thin? You might remember hearing any number of horror stories of an over-bored cylinder with too thin a cylinder wall. This was far more common during the Shovelhead days when big bore meant exactly that.

Take the stock cylinder and bore into it. 0.020” over sure, .040” over why not, there’s even .060” and every so often you find a .070” over set of pistons. That’s just asking for trouble. Crazy as it may sound, but cylinders can crack and you don’t want to be anywhere near that type of mess. Good night, Irene. Good bye, Charlie. That poor motor might even get turned down as scrap metal.

Okay, so we know that you can’t go too thin. Well, what’s too thin? That is the million dollar question.

One of the first big bore kits to come out for the Twin Cam was the S&S 95 ci big bore kit with 3.875” bore cylinders. I went to this upgrade when the camshaft needle bearings blew on my 2000 Road King, but camshafts will have to wait.

Since the 95 kit came out, and is now obsolete, S&S has provided a 97 ci big bore kit (only considering cylinders at this point, larger upgrades are possible when changing the stroke), and now 98 ci for the 1999-2006 Twin Cams. The same cylinder size can be used on the 2007-2015 Twin Cams (different kit due to piston skirt length to avoid contact from longer stroke). The 106 ci kit and 107 ci kits are different bores, 3.927” and 3.937”.

What’s to stop the next size larger? I think it might be a stretch on the bore department for stock-style cylinders.

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What’s to stop the next size larger? I think it might be a stretch on the bore department for stock-style cylinders.

Larger bolt-on bores are possible with the Revolution Performance line of cylinder and piston kits that use nickel silicon carbide electroplated bores. This type of technology offers a host of benefits, including better heat displacement and long life, but eliminates the possibility of enlarging the cylinder due to the hardness of the coating that needs special diamond-plated honing tools.

Given the S&S kits, the one cubic-inch gain from what equates to a 0.010” over bore from the 106 ci kit, S&S went with an upgrade from the standard piston to a CP Piston. These high-end pistons provide the combined technology and quality to push the envelope a little farther. The major limiting factor is in the spigot end of the cylinder that goes into the crank case.

JIMS offers a line of 4” and 4-1/8” cylinder and piston kits that will reuse the flywheel but require boring of the crank case. This extra labor allows a larger bore. But once you go down that road, why not get into a flywheel upgrade which we will cover in a later blog.

And once going down that rabbit hole, you start into the idea of once you’re in there, why don’t I start changing this and that. Anyone who has been around racing will tell you, you can always get a little more performance out of a motor. The question is just how much do you want to invest to get a couple more points of horsepower.

97 ci Silver Big Bore Kit

So before we end this blog entry, take this thought with you: The biggest bang for the buck in the engine game is displacement. There is no replacement for displacement. There are tons of ways to get you down the road, but when you want to rip past that 18-wheeler on the highway, or use that new rider's backrest as a back stop, take a look at our top-end cylinder and piston kits for your motorcycle.

We carry S&S Cycle, Revolution Performance, JIMS, RevTech and V-Twin Manufacturing cylinder and piston kits. Most kits listed are bolt-on but some do require case boring. All installation does require mechanical knowledge and at least standard mechanics tools, but if you can change a set of head gaskets, you can change your bore size. The S&S Cycle big bore kits listed in this blog come with cylinders, pistons, and head and base gaskets.

I recommend reading through your service manual to familiarize yourself with the procedure on head gasket replacement before attempting. You will need to replace your rocker gaskets in the process. To get the most out of the big bore, you might have to adjust your air-fuel ratio. The carburetor might need a bit more fuel, or you might have to fine tune your fuel injection. If you’ve changed your exhaust and air cleaner you should be familiar with these settings and it’ll be even easier the second time around.